JPH0860292A - High-strength steel with excellent toughness - Google Patents

Abstract

(57) [Summary] [Purpose] To provide high-strength steel with excellent weld heat-affected zone (HAZ) toughness. [Constitution] (1) C: 0.03 to 0.15%, Si: 0.01 to 0.5%, M
n: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.015
%, Al: 0.0005 to 0.05%, N: 0.005 to 0.02%, P in the impurities is 0.030% or less, and S is 0.030% or less, a high-strength steel excellent in HAZ toughness. This steel has Cu: 0.2
~ 1.5%, Ni: 0.2-3.0%, one or two kinds, and / or Cr: 0.05-1.0%, Mo: 0.05-1.0%, V: 0.03
.About.0.2%, B: 0.0003 to 0.002%, or one or more kinds thereof may be contained. In addition, Ca: 0.0005 to 0.
005%, REM: 0.005 to 0.05%, and may contain one or two kinds. [Effect] It is possible to secure high HAZ toughness as well as high base metal toughness. With this steel material, the low temperature toughness of the welded structure is further improved, and the high heat input welding method can be applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having excellent weld heat-affected zone toughness, which is used for welded structures such as pressure vessels, ships, bridges, buildings, offshore structures and line pipes.

[0002]

2. Description of the Related Art In recent years, in high-strength steel for welding used for offshore structures installed in ice sea areas, line pipes for cold regions, or large-scale structures such as ships and LNG tanks, improvement in material properties has been achieved. Demands are becoming strict, and sufficient strength is required according to the purpose of use, and in particular, there is a high demand for toughness improvement in the weld heat affected zone (hereinafter referred to as HAZ) of the base material in contact with the weld metal. .

Conventionally, the weld toughness of high-strength steel sheets has been
It is known that austenite (hereinafter referred to as γ) crystal grain size, transformation structure, precipitation state of fine hardened phase and amount of solute N in steel sheet have a great influence, and various measures for improving toughness of weld zone are taken. Has been proposed.

[0006] For example, regarding the above and the above, a method of suppressing the coarsening of γ crystal grains by adding a small amount of Ti and finely precipitating TiN in the steel (“Iron and Steel” No. 1 issued in June 1979)
Vol. 65, No. 8, page 1232) and CaS and
A method of refining the structure by producing CaO and refining the γ crystal grains and precipitating ferrite (hereinafter referred to as α) in the grains with CaS and CaO as nuclei (“February 1983”). Journal of Welding Society, Vol. 52, No. 2, p. 49), rare earths (hereinafter R
Similarly, a method of refining crystal grains with an oxide of an element (referred to as EM) (refer to Japanese Patent Laid-Open No. 64-15320) is used, in which intra-grain α is generated using Ti oxide particles as nucleation sites to refine the structure. (JP-A-57-51243 and JP-A-61-79)
No. 745), and a method in which V and Ti that are precipitated in the cooling process are used as α transformation nuclei by adding V and Ti in combination (see JP-A-5-186848).

With respect to the above, there has been proposed a method of suppressing precipitation of a hardening phase by lowering carbon equivalent or reducing Si and Al (see Japanese Patent Laid-Open No. 2-1904213).

With respect to the above, there have been proposed methods such as a method of reducing the amount of N contained in steel and a method of fixing N as AlN by adding excess Al.

[0007] However, in the above measures, TiN
Is believed to be mostly dissolved in the base metal in the portion heated to 1400 ° C. or higher, and in particular, coarsening of γ crystal grains in the vicinity of the high heat input welding HAZ fusion line is unavoidable. Further, TiN dissolved in the heating process does not reprecipitate in the cooling process. That is, in the portion where TiN is melted, there is a drawback that the α-transformation does not occur in the grains during the cooling process and further the amount of solute nitrogen is increased, so that the deterioration of HAZ toughness cannot be avoided.

On the other hand, with regard to the use of Ca, REM and Ti oxide particles, it is very difficult to disperse them in steel in a finely stable state during melting, and it is practically practical. The problem remains.

Techniques such as controlling the precipitation morphology of the finely hardened phase and reducing the amount of solute N by methods such as low carbon equivalent, low Si and low Al are synergistic with the improvement of toughness by the above-mentioned nitrides or oxides. This is aimed at the effect, and the effect of improving the HAZ toughness by itself is naturally limited.

Nb is a very important element along with Ti and V from the viewpoint of securing the strength of steel.
Various methods have been proposed for improving the strength and toughness of the steel base material by addition.

Japanese Patent Publication No. 56-50779 and Japanese Patent Publication No. 57-
Japanese Patent No. 19734 discloses a method of obtaining a high toughness steel material by specifying a trace amount of Nb content and rolling and heat treatment methods to make the base metal structure fine α. JP-A-6-49586
According to the publication, Nb content provides an effect of suppressing γ grain growth during heating in the manufacturing process of a steel sheet, an effect of strengthening NbC precipitation in a deformation zone during rolling, and an effect of preventing HAZ softening during high heat input welding. There is a description.

However, since the stability of Nb carbides and nitrides at high temperatures is extremely lower than that of TiN, HA containing Nb is
There are almost no proposals for improving Z toughness from the viewpoint of precipitates, and the present state is that Nb content in HAZ toughness-improving steel is made exclusively for the purpose of securing the strength of the base metal (for example, JP (See Sho 59-35619).

[0013]

The present invention has been made to solve the above problems, and an object of the present invention is to perform welding under a wide heat input condition from medium heat input to large heat input. On the other hand, it is an object of the present invention to provide a high-strength steel for welding, which has a fine structure even in a region near the melting line where it is heated to 1400 ° C. or higher and stably exhibits good low temperature HAZ toughness.

[0014]

The summary of the present invention is as follows.
It is a high-strength steel having excellent HAZ toughness of (1) to (5).

(1) C: 0.03 to 0.15% by weight, Si: 0.0
1 to 0.5%, Mn: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti:
0.005 to 0.015%, Al: 0.0005 to 0.05% and N: 0.005
A high-strength steel excellent in HAZ toughness, characterized in that the content of Fe is 0.030%, the balance is Fe and unavoidable impurities, and P is 0.030% or less and S is 0.030% or less.

(2) Excellent HAZ toughness, characterized in that, in addition to the component of (1) above, it further contains one or two of Cu: 0.2 to 1.5% and Ni: 0.2 to 3.0% by weight. High strength steel.

(3) In addition to the component of (1) above, further, in weight%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, V: 0.03.
A high-strength steel excellent in HAZ toughness, characterized in that it contains one or more of 0.2 to 0.2% and B: 0.0003 to 0.002%.

(4) In addition to the above-mentioned component (1), in addition, one or two of Cu: 0.2 to 1.5% and Ni: 0.2 to 3.0% by weight, Cr: 0.05 to 1.0%, Mo: 0.05. ~ 1.0%,
High-strength steel with excellent HAZ toughness, characterized by containing one or more of V: 0.03 to 0.2% and B: 0.0003 to 0.002%.

(5) In addition, in% by weight, Ca: 0.0005
~ 0.005% and REM: 0.005 to 0.05% of one or two kinds, a high-strength steel excellent in HAZ toughness according to any one of the above (1) to (4).

The present inventors have conducted a study focusing on the solid solution of nitride in the base metal in the vicinity of the welding line and the reprecipitation during the welding cooling process, and refine the γ grains and reduce the amount of solid solution elements in the HAZ. A comparison was made on the toughness improving action. As a result, the following new findings were obtained.

When a trace amount of Ti is added to Nb-containing steel, extremely fine and large amount of Nb-Ti composite carbonitride is precipitated.

This composite carbonitride is relatively stable even when heated to 1400 ° C. or higher, and has a sufficient γ-grain refining effect.

By increasing the amount of N in the steel,
The composite carbonitride stabilizes and has the effect of substantially reducing the amount of solid solution elements.

When the content of Ti is excessive with respect to the content of Nb, the composite carbonitride is not formed, and the Ti nitride alone precipitates. Therefore, the content of Ti and Nb is limited. What you need to do.

Further, the problems of the prior art utilizing Nb precipitates were investigated, and in order to effectively utilize Nb precipitates for HAZ toughness, a small amount of Ti was added to Nb-containing steel in combination. Forming a stable Nb-Ti composite carbonitride,
Furthermore, it was experimentally clarified that it is important to limit the content of N, and the HAZ toughness can be dramatically improved by these methods. This will be described with reference to FIG.

FIG. 1 shows 0.05% C-0.10% Si-1.3% Mn-
Nb-containing steel based on 0.002% P-0.003% S,
In Ti-containing steel and Nb-Ti-containing steel, when the N content was changed, the HAZ toughness when Charpy was applied at a maximum heating temperature of 1400 ° C and a heat cycle simulating large heat input welding equivalent to 100 kJ / cm It is a figure which shows the result evaluated as a transition temperature in an impact test.

As shown in FIG. 1, it is understood that the HAZ toughness is greatly improved by the addition of the Nb-Ti composite. That is, in the Nb-containing steel, the HAZ toughness deteriorates when the N content increases, whereas in the Ti-containing steel, the HAZ toughness improves as the N content increases up to about 0.004%, and the N content increases to 0.005%.
%, The HAZ toughness deteriorates. Further, in the Nb-Ti composite added steel, the HAZ toughness is improved as the N content is increased, and particularly when the N content is 0.005% or more, the HAZ toughness is dramatically improved.

The reason for the improvement of the HAZ toughness is γ due to the fact that the Nb precipitate is compounded with Ti and the precipitate is stabilized.
This is to improve the grain refining effect and reduce the solid solution element.

Further, the γ grain size becomes finer as the amount of N increases. It was easily expected that this was caused by the increase in the amount of precipitation of the nitride system due to the increase of the amount of N, but from the detailed investigation results of the precipitation elements, the increase in the amount of N It was found that not only the amount of precipitates was increased, but also the NbC component in the Ti-Nb composite carbonitride was increased.

That is, in the steel of the present invention, an increase in the amount of N stabilizes not only nitrides but also carbides at high temperatures, thereby increasing the amount of precipitates and strengthening the refinement of the γ grain size. It is considered that the molten Ti and the solid solution Nb are reduced, the hardenability is reduced, and the HAZ toughness is improved.

[0031]

The reason why the chemical composition of the steel of the present invention is determined as described above will be explained below. % Means% by weight.

C: 0.03 to 0.15% C is an element which has the effects of ensuring strength and forming a Nb-Ti composite carbonitride to refine the structure. If the C content is less than 0.03%, these effects cannot be obtained. On the other hand, if it exceeds 0.15%, martensite (α ') will appear in the weld.
And similar pearlite (α / Fe ₃ C) are generated to deteriorate the HAZ toughness and adversely affect the toughness and weldability of the base material. Therefore, the range of the C content is 0.03 to 0.15%.

Si: 0.01 to 0.5% Si is an effective element for preliminary deoxidation of molten steel, and for this reason, it is contained in steel in an amount of about 0.01%. On the other hand, since Si does not form a solid solution in cementite, if the Si content exceeds 0.5%, it prevents the untransformed γ grains from decomposing into α grains and cementite, and the formation of island-shaped martensite, which is a fine hardening phase. To deteriorate the HAZ toughness. Therefore, the range of Si content is set to 0.01 to 0.5%.

Mn: 0.4-2.0% Mn is an element necessary for ensuring strength and toughness. To obtain these effects, it is necessary to set the Mn content to 0.4% or more. However, if the Mn content exceeds 2.0%, the hardenability is increased, and the weldability and HAZ toughness are deteriorated. Therefore, the range of Mn content is set to 0.4 to 2.0%.

Nb: 0.01 to 0.1% Nb is very effective from the viewpoint of securing strength, and
It is an essential element for forming a composite carbonitride together with Ti and increasing the HAZ toughness. These effects cannot be obtained when the Nb content is less than 0.01%. On the other hand, if it exceeds 0.1%, coarse NbC is independently precipitated, which is harmful to the toughness of the base material. Therefore, the range of Nb content is 0.01 to 0.1%.

Ti: 0.005 to 0.015% Ti is an essential element for forming a composite carbonitride together with Nb. To obtain this effect 0.005% or more
It must be Ti content. On the other hand, the Ti content is 0.01
If it exceeds 5%, it causes precipitation of coarse TiC by itself, and HA
It is detrimental to Z and the toughness of the base metal. Therefore, the Ti content range is 0.005 to 0.015%.

Al: 0.0005 to 0.05% Al is an element necessary as a deoxidizing agent. To obtain the deoxidizing effect, an Al content of 0.0005% or more is required. On the other hand, 0.05
%, The formation of island martensite is promoted similarly to Si, which hinders the improvement of HAZ toughness. Therefore, the upper limit is set to 0.05%.

N: 0.005-0.02% N is an element necessary for producing Nb-Ti composite carbonitride. As shown in FIG. 1, a N content of 0.005% or more is required from the viewpoint of improving the HAZ toughness.

However, if it exceeds 0.02%, the toughness of the base metal is lowered, the toughness of the weld metal is lowered due to mixing into the weld metal due to dilution during welding, and it is also undesirable for preventing weld cracking. The upper limit was 0.02%.

P: 0.030% or less P is an unavoidable impurity element that causes grain boundary segregation, and therefore causes grain boundary cracking in the HAZ. For this reason,
The lower the P content is, the more preferable it is, but the upper limit was set to 0.030% in consideration of the economical viewpoint. In order to improve both the base material toughness and the HAZ toughness and reduce the slab center segregation, 0.01% or less is desirable.

S: 0.030% or less When a large amount of S is present, it forms a precipitate such as MnS which is a starting point of welding cracks. Therefore, the smaller the content, the more desirable. However, considering economic efficiency, the upper limit was set to 0.030%. In order to improve both the base material toughness and the HAZ toughness and reduce the slab center segregation, 0.01% or less is desirable.

The steel of the present invention can further contain one or two of the following Cu and Ni in addition to the above base composition.

Cu: 0.2 to 1.5% Cu is an effective element for securing the strength and toughness of the base material. These effects cannot be obtained when the Cu content is less than 0.2%. On the other hand, if it exceeds 1.5%, on the contrary, the strength and toughness of the base material decrease.

Ni: 0.2 to 3.0% Ni, like Cu, is an element effective for securing the strength and toughness of the base material. These effects cannot be obtained when the Ni content is less than 0.2%. On the other hand, if it exceeds 3.0%, on the contrary, the strength and toughness of the base material are lowered.

When Cu and Ni are contained in combination, it is desirable that their total content be 3.0% or less. More desirable is 2% or less. As a result, it is possible to avoid an excessive increase in hardenability due to the inclusion of a large amount of a single substance, minimize the adverse effect on HAZ toughness and weldability, and obtain a desired balance between strength and toughness.

The steel of the present invention may further contain one or more of the following Cr, Mo, V and B in addition to the above-mentioned base composition. All of these elements are effective in increasing the hardenability of the steel material and ensuring the strength. When two or more kinds are contained, the total content is preferably 2% or less. More desirable is 1.5% or less. With these, similarly, it is possible to avoid an excessive increase in hardenability due to the inclusion of a large amount of a single substance, minimize the adverse effect on HAZ toughness and weldability, and obtain a desired balance between strength and toughness.

Cr: 0.05 to 1.0% If the Cr content is less than 0.05%, the above effect cannot be obtained. On the other hand, if it exceeds 1.0%, it is not preferable from the viewpoint of causing deterioration of the toughness of the base material and HAZ and suppressing hardening of HAZ and cold cracking in welding.

Mo: 0.05 to 1.0% For the same reason as Cr, the content range is set to 0.05 to 1.0%.

V: 0.03 to 0.2% If the V content is less than 0.03%, the effect of improving hardenability and strength cannot be obtained. On the other hand, if it exceeds 0.2%, the toughness of the base material and HAZ is deteriorated, and it is not preferable from the viewpoint of suppressing hardening of HAZ and cold cracking in welding.

B: 0.0003 to 0.002% If the B content is less than 0.0003%, the effect of improving hardenability and strength cannot be obtained. On the other hand, if it exceeds 0.002%, the base metal and H
It is not preferable from the viewpoint of causing deterioration of the toughness of AZ and suppressing hardening of HAZ and cold cracking in welding.

In the steel of the present invention, in addition to the above base composition, one or more of the above-mentioned Cu and Ni element groups and one or more of the element groups from Cr to B are respectively selected. May be contained at the same time. The desirable total amount and effect of each element group are as described above.

In any of the above-mentioned steels of the present invention, one or two of the following Ca and REM can be contained in addition to the above in an amount of% by weight. These control the morphology of sulfides, improve the low temperature toughness, and have the effect of improving the hydrogen-induced cracking resistance.

Ca: 0.0005 to 0.005% If the Ca content is less than 0.0005%, the above effect cannot be obtained. On the other hand, if it exceeds 0.005%, large inclusions and clusters are formed to impair the cleanliness of steel.

REM: 0.005 to 0.05% If REM is less than 0.005%, the above effect cannot be obtained.
On the other hand, if it exceeds 0.05%, large inclusions and clusters are formed to impair the cleanliness of steel. As REM, Ce, La, Y
It is desirable to use one kind or two or more kinds of Hf and Hf, and the above effects can be obtained in any case.

In the production of the high-strength steel of the present invention, the steel of the above component system is melted in a converter, an electric furnace, etc., and is made into a slab by continuous casting or ingot agglomeration. At this time, a higher cooling rate is preferable from the viewpoint of finely dispersing the precipitates, so it is preferable to use the continuous casting method. Further, it is desirable that the slab thickness is thin for the same reason.

Then, heating and hot rolling are performed to manufacture a predetermined steel sheet.

Even if various known techniques such as controlled rolling and controlled cooling are applied as the manufacturing conditions after the above heating, the HAZ
Has no effect on the nature of. However, if reheating is performed during hot rolling, the slab reheating temperature is 1250.
It is desirable to set the temperature below ℃.

Although the Nb-Ti composite carbonitride is finely precipitated in the slab, it is not a short heat cycle such as the welding heat treatment cycle, and therefore, the slab re-temperature exceeds 1250 ° C. This is because when heating is performed, it is not possible to suppress the coarsening of γ crystal grains, and the low temperature toughness of the base material deteriorates.

Further, in order to improve the mechanical properties of the base material, even if an appropriate heat treatment is applied after the hot rolling, there is no problem with respect to the HAZ toughness.

In the above manufacturing method, it is not always necessary to reheat the slab, and hot rolling or direct rolling does not impair the characteristics of the steel of the present invention.

[0061]

EXAMPLES Steels having the chemical compositions shown in Tables 1 to 3 were melted in a converter, made into slabs by the ingot-agglomeration method or continuous casting method, and then heated to 1150 to 1250 ° C. and hot rolled. And then Table 4 ~
The heat treatment shown in Table 6 was performed to manufacture a steel plate having a plate thickness of 20 to 60 mm.

The mechanical properties of the base material and the HAZ toughness of the welded portion were investigated for these steel sheets. The method of investigating the HAZ toughness is as follows.

Reproduction HAZ1: Maximum heating temperature corresponding to welding heat input of 100 kJ / cm, 1400 ° C., cooling time of 800 to 500 ° C.
Charpy impact test is conducted after 60 second heat cycle.

Reproduction HAZ2: Maximum heating temperature corresponding to welding heat input amount of 250 kJ / cm, 1400 ° C., cooling time of 800 to 500 ° C.
Charpy impact test is conducted after 180 seconds heat cycle. Welded joints: For some steel grades, actual joints with a welding heat input of 100 kJ / cm were produced by the SAW method, and a Charpy impact test was conducted.

The results of these tests are also shown in Tables 4 to 6. In FL of Table 4, HAZ near the fusion line and weld metal are 1:
The WM means a Charpy test when a notch is formed at a position of 1, and the WM is a Charpy test when a notch is formed at the center of the weld metal.

[0066]

[Table 1]

[0067]

[Table 2]

[0068]

[Table 3]

[0069]

[Table 4]

[0070]

[Table 5]

[0071]

[Table 6]

Steels 1 to 24 in Table 1 are steel types whose reproduced HAZ toughness is shown in FIG. In Steels 1 to 3, the N content is insufficient, and complex carbonitrides are not sufficiently precipitated. Therefore, the toughness of the reproduced HAZ does not have a high value. In Steel 10, the reproduced HAZ toughness value is excellent because the N content is excessive, but the toughness of the base material and the weld metal toughness of the actual joint are significantly deteriorated.

Steels 11 to 17 are steels containing Nb alone, but in this case, almost all the Nb precipitates form a solid solution in the base metal by undergoing a welding reproducible heat cycle. Therefore, the increase in the N content directly causes an increase in the amount of solid solution N, and the reproduced HAZ toughness deteriorates as the N content increases. Steel 18-24 is Ti
Although it is a single content steel, in this case, some improvement in toughness is observed as the N content increases. However, since only TiN can be expected as a precipitate, the reproduced HAZ toughness rapidly deteriorates when the ratio of Ti content to N content is about 3.4. Further, it is clear from FIG. 1 that the margin for improving the toughness of the steel containing only Ti is smaller than that of the Ti-Nb composite added steel.

The steels 4 to 9 of the present invention show excellent HAZ toughness as compared with the above-mentioned comparative steels, and also excellent in base material toughness, base material mechanical properties, and weld zone toughness during welding.

In Comparative Steels 26 to 33, the HAZ toughness is remarkably deteriorated because some of the components deviate from the ranges defined by the present invention.

The steels 34 to 61 of the present invention contain elements having an action of improving the strength and toughness of the base material, and all show excellent reproduced HAZ toughness.

The steels 62 to 64 of the present invention were obtained by examining the influence of the difference in the heat treatment method in the case of the components satisfying the present invention. When compared with the HAZ toughness of inventive steel 5, it is clear that the change in heat treatment after rolling has no effect on the reproduced HAZ toughness.

[0078]

EFFECTS OF THE INVENTION The high-strength steel for welding of the present invention can ensure high base metal toughness as well as excellent HAZ toughness. With this steel material, the low temperature toughness of the welded structure is further improved, and the high heat input welding method can be applied. As a result, the weldability and the stability of the welded structure can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of N content on the transition temperature in the case of Nb alone, Ti alone, and Ti—Nb composite contained steels.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication C22C 38/58

Claims (5)

[Claims] 1. By weight%, C: 0.03 to 0.15%, Si: 0.01 to
0.5%, Mn: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti: 0.00
5 to 0.015%, Al: 0.0005 to 0.05% and N: 0.005 to 0.
A high-strength steel with excellent toughness in the weld heat-affected zone, containing 02% and the balance Fe and unavoidable impurities, with P in the impurities being 0.030% or less and S being 0.030% or less. 2. By weight%, C: 0.03 to 0.15%, Si: 0.01 to
0.5%, Mn: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti: 0.00
5 to 0.015%, Al: 0.0005 to 0.05% and N: 0.005 to 0.
02%, Cu: 0.2-1.5% and Ni: 0.2
~ 3.0% of one or two kinds, the balance consisting of Fe and unavoidable impurities, P in the impurities is 0.030% or less, S
Is a high-strength steel with excellent toughness in the weld heat-affected zone, characterized by a content of 0.030% or less. 3. By weight%, C: 0.03 to 0.15%, Si: 0.01 to
0.5%, Mn: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti: 0.00
5 to 0.015%, Al: 0.0005 to 0.05% and N: 0.005 to 0.
02%, Cr: 0.05-1.0%, Mo: 0.05-1.
0%, V: 0.03 to 0.2% and B: 0.0003 to 0.002%, and one or more of them, the balance consisting of Fe and inevitable impurities, P in the impurities is 0.030% or less, S is 0.03% or less.
A high-strength steel having excellent toughness in the weld heat-affected zone, which is characterized by being 0% or less. 4. C: 0.03 to 0.15% by weight, Si: 0.01 to
0.5%, Mn: 0.4 to 2.0%, Nb: 0.01 to 0.1%, Ti: 0.00
5 to 0.015%, Al: 0.0005 to 0.05% and N: 0.005 to 0.
02%, Cu: 0.2-1.5% and Ni: 0.2
~ 3.0% of 1 or 2 kinds, Cr: 0.05 ~ 1.0%, Mo: 0.
05-1.0%, V: 0.03-0.2% and B: 0.0003-0.00
1% or 2% or more of 2%, the balance Fe and unavoidable impurities, P in the impurities is 0.030% or less, S
Is a high-tensile steel with excellent toughness in the weld heat-affected zone, characterized by a content of 0.030% or less. 5. In addition, in a further weight%, Ca: 0.0005 to 0.00
5% and rare earths: 0.005 to 0.05% of one or two kinds are included, and the high tensile steel excellent in toughness of the weld heat affected zone according to any one of claims 1 to 4.